The invention relates generally to multiphase transformers. More particularly, the invention relates to multiphase transformer core and winding designs for reduced harmonic currents in the primary windings.
Transformers are commonly used in many applications for converting AC voltage, from a variable source such as an alternator, to a different AC voltage level, or in combination with a rectifier circuit to DC voltage. For example, in the aerospace industry, aircraft control systems often use electrical servo and tractive motors that require different voltage levels from the source voltage. Many of these uses require transformers to supply voltage to rectifier circuits delivering 90 VDC to 440 VDC.
The primary energy source in such applications is typically an aircraft alternator that produces three phase outputs at an operating frequency in the typical range of 400 to 800 hertz depending on engine speed and loads. These alternators may be used for supplying power to a wide variety of DC circuits and loads. Accordingly, aerospace standards (such as MIL-STD-1399) typically require that there be no harmonic currents drawn from the alternator main power lines which exceed 3% (-30 db) of the rated fundamental current up to the 32nd harmonic. It is expected that these types of limits on harmonic currents drawn from the main supply also will apply in general to European Economic Community members for industrial and consumer AC/DC power conversion equipment starting in 1993.
A typical uncompensated 3 phase full wave bridge or 6 phase half wave bridge AC/DC rectifier generates well above 10% harmonic currents.
Known methods and apparatus for reducing these harmonic currents include a minimum use of 24 rectifiers connected to twelve multiphase secondary windings with the use of additional chokes and interphase transformers being required to meet the 3% harmonic current requirement. Other approaches include power factor correction circuits which use pulse width modulated regulator circuitry to control harmonics. These designs involve complicated and expensive circuits and controls.
The need exists, therefore, for a three phase input transformer design that produces reduced harmonic currents, such as in an AC/DC conversion circuit, in a package that is weight and cost competitive with the aforementioned known systems.